The growth of 16 ram lambs was severely restricted for the first 6 weeks of post-natal life. Subsequently, these lambs (group R) were fed ad libitum. The diet was based on reconstituted cows' whole milk and lucerne chaff. A control (group C) of 16 similar lambs was fed ad libitum on the same diet from birth.
Lambs were weighed regularly and, in group C, four lambs were killed at the age of 1 day and then two at each of the following body weights: 10, 15, 20, 25, 30, 35 kg. In group R, five lambs were killed at the commencement of ad libitum feeding (age 43 days) and two each at the same body weights as in group C, except that only one lamb was available at 20 kg. After slaughter, the brain, liver, kidneys and the semitendinosus and gastrocnemius muscles were removed, weighed, stored and, with the exception of the liver, were analysed later for the following components: DNA, RNA and protein. Carcass weight and the weight of the kidney and channel (KC) fat were recorded. The femurs and metacarpals were removed from each carcass and cleaned, weighed and measured.
During the 6 weeks of restricted feeding, group R gained 0·9 kg while the ad libitum group C gained 13·5 kg. However, during recovery, group R grew faster than group C (0·37 ν 0·34 kg/day), reducing the weight for age difference near the end to 29 days at mean body weights of 30 kg.
Because of the design of the experiment, at the same age all measurements on group R animals, after the commencement of ad libitum feeding, were smaller than in group C. For this reason, the interpretation of the results has been based on differences between regression equations relating the various measurements to empty-body weight or to one another.
At the start of ad libitum feeding, brain weight, carcass weight, femur weight and femur length were bigger, while liver weight and KC fat weight were smaller in group R than in group C. At the end of the experiment, there were no significant differences between treatments for these measurements. Metacarpal shape differed between groups, the bone being relatively longer and narrower in group R than in group C, throughout the period of ad libitum feeding.
There were no significant differences between treatments in the relationships between DNA and corresponding tissue weights. However, the RNA was significantly less in both muscles in group R than in group C at the beginning of ad libitum feeding, but this difference had disappeared by the end of the experiment.
The brain protein: DNA and the brain weight: DNA ratios did not differ between treatments nor did they change significantly during the experiment. The semitendinosus was the only other tissue for which protein content was available and the protein: DNA ratio for this muscle differed between treatments, reflecting an acceleration of division by cell nuclei during the recovery period. The other tissue weight: DNA relationships did not differ between treatments and all ratios increased to values similar to those reported elsewhere. RNA:DNA ratios differed between treatments for both muscles, suggesting that high rates of protein synthesis occurred in group R during the recovery period.
In spite of the apparent normality of group R when measurements were related to EBW or tissue weights at the end of the experiment, only a long-term investigation would determine whether the weight-for-age difference of the type reported here would persist in adult life.